Derivatives of 6-fluorophenanthridine for liquid crystal mixtures

ABSTRACT

5-Fluorophenanthridine derivatives and their use in liquid-crystalline mixtures 6-Fluorophenanthridine derivatives of the formula (I) ##STR1## in which G is --CF═N--, 
     E 1 , E 2 , E 3 , E 4 , E 5  and E 6  are identical or different and are --CH--, --CF-- or --N-- 
     and R 1  (--A 1  --M 1 ) a  (--A 2  --M 2 ) b , (M 3  --A 3  --) c  (M 4  --A 4  --) d  R 2  denote mesogenic radicals 
     are suitable as components for liquid-crystalline mixtures, especially ferroelectric mixtures.

In addition to nematic and cholesteric liquid crystals, recent timeshave also seen the use of optically active tilted smectic(ferroelectric) liquid crystals in commercial display devices.

Clark and Lagerwall showed that the use of ferroelectric liquid crystals(FLCs) in very thin cells leads to optoelectric switching or displayelements having response times faster by a factor of up to 1000 thanthose of the conventional TN (twisted nematic) cells (see, for example,EP-A 0 032 362). On the basis of these and other favorable properties,for example the possibility of bistable switching and the contrast,which is virtually independent of viewing angle, FLCs are in principlehighly suited to applications such as computer displays.

For the use of FLCs in electrooptical or completely optical assembliesthere is a need either for compounds which form tilted or orthogonalsmectic phases and which are themselves optically active, or else forcompounds which, although forming such smectic phases are not themselvesoptically active, can be doped with optically active compounds to induceferroelectric smectic phases. The desired phase should at the same timebe stable over as wide as possible a temperature range.

Obtaining a good contrast ratio in electrooptical assembliesnecessitates a uniform planar orientation of the liquid crystals. Goodorientation in the S_(A) and S^(*) _(C) phase can be achieved, forexample, when the phase sequence of the liquid-crystal mixture withdecreasing temperature is as follows:

isotropic→N^(*) →S_(A) →S^(*) _(C)

A precondition is that the pitch of the helix in the N^(*) phase is verylarge (greater than 10 μm) or, even better, is fully compensated (see,for example: T. Matsumoto et al., Proc. of the 6th Int. Display ResearchConf., Japan Display, Sept. 30-Oct. 2, 1986, Tokyo, Japan, pp. 468-470;M. Murakami et al., ibid., pp. 344-347). This is done, for example, byadding one or more optically active dopes which induce, say, aright-handed helix to the chiral liquid-crystal mixture which in theN^(*) phase has a left-handed helix, in amounts such that the helix iscompensated.

For the use of the SSFLCD effect (Surface Stabilized FerroelectricLiquid Crystal Display) of Clark and Lagerwall for uniform planarorientation a further precondition is that the pitch in the smecticC^(*) phase is substantially greater than the thickness of the displayelement (Mol. Cryst. Liq. Cryst. 1983, 94, 213 and 1984, 114, 151).

The optical response time τ[μs] of ferroelectric liquid-crystal systems,which should be as short as possible, depends on the rotationalviscosity of the system γ[mPas], on the spontaneous polarization P_(s)[nC/cm² ] and on the electric field strength E [V/m] in accordance withthe relationship ##EQU1##

Since the field strength E is determined by the electrode separation inthe electrooptical component and by the applied voltage, theferroelectric display medium must be of low viscosity and must have ahigh spontaneous polarization in order for a short response time to beobtained.

Finally, requirements in addition to thermal, chemical and photochemicalstability are for a small optical anisotropy Δn and a small positive or,preferably, negative dielectric anisotropy Δε (see, for example: S. T.Lagerwall et al., "Ferroelectric Liquid Crystals for Displays", SIDSymposium, Meeting October 1985, San Diego, Calif., USA).

The entirety of these requirements can only be met with mixtures of twoor more components. The basis of these mixtures (or matrix) ispreferably formed by compounds which as far as possible themselvesalready have the desired phase sequence I→N→S_(A) →S_(C). Furthercomponents are often added to the mixture in order to lower the meltingpoint and to broaden the S_(C) phase and usually the N phase as well,for inducing the optical activity, for pitch compensation and foradapting the optical and dielectric anisotropy, as far as possiblewithout increasing the rotational viscosity, for example.

Ferroelectric liquid-crystal displays can also be operated by utilizingthe DHF (Distorted Helix Formation) effect or the PSFLCD (PitchStabilized Ferroelectric Liquid Crystal Display, also called SBF=ShortPitch Bistable Ferroelectric) effect. The DHF effect was described by B.I. Ostrovski in Advances in Liquid Crystal Research and Applications,Oxford/Budapest, 1980, 469 ff. and the PSFLCD effect is described inDE-A 39 20 625 and EP-A 0 405 346. In contrast to the SSFLCD effect,utilizing these effects requires a liquid-crystalline material with ashort S_(C) pitch.

Phenanthridine derivatives fluorinated in positions 4 and/or 7 are knownas liquid crystals or as components of liquid-crystalline mixtures fromDE-A 44 02 986.

Derivatives of phenanthrene and of 9,10-dihydrophenanthrene have alreadybeen described as liquid crystals or as components of liquid-crystallinemixtures: Azomethines having a phenanthrene or 9,10-dihydrophenanthreneunit are known from J. Chem. Soc. 1958, 552 and J. Chem. Soc., PerkinTrans. II 1982, 465-472; keto derivatives of 9,10-dihydrophenanthreneare known from Chem. Ind. [London] 1974, 615, Prod. Int. Liq. Cryst.Conf. 1973, 397 and Tetrahedron 1981, 37, 2815; carboxyl derivatives of9,10-dihydrophenanthrene are known from DD-WP 153 826 and Z. Chem. 1983,23, 21-22; 2,7-bis(alkoxy)phenanthrenes are known from Nippon KagakuKaishi 1980, 250; and phenanthrene derivatives having one or morefluorine substituents in positions 1, 3, 4, 5, 6 and 8 are known fromDE-A 19 500 768.

However, since the development--of ferroelectric liquid-crystal mixturesin particular--can in no way be regarded as complete, the manufacturersof displays are interested in a very wide variety of components formixtures. One of the reasons for this is that only the interaction ofthe liquid-crystalline mixtures with the individual components of thedisplay device or of the cells (for example the alignment layer) allowsconclusions to be drawn about the quality of the liquid-crystallinemixtures too.

The object of the present invention was therefore to provide compoundswhich are suitable in liquid-crystalline mixtures for improving theprofile of properties of these mixtures.

It has now been found, surprisingly, that 6-fluorophenanthridinederivatives of the formula (I) are particularly suitable for use inliquid-crystal mixtures.

The invention therefore provides compounds of the formula (I) ##STR2##in which the symbols and indices have the following meanings: G is--CF═N--.

E¹, E², E³, E⁴, E⁵ and E⁶ are identical or different and are --CH--,--CF-- or --N--, with the proviso that E⁴ and E⁵ cannot simultaneouslybe --N--;

R¹ and R² are identical or different and are

a) hydrogen, --F, --Cl, --CF₃, --OCF₃ or --CN,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 20 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, --S--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH₃)₂ --,and/or

b2) one or more CH₂ groups can be replaced by --CH═CH--, --C.tbd.C--,cyclopropane-1,2-diyl, 1,4-phenylene, 1,4-cyclohexylene or1,3-cyclopentylene, and/or

b3) one or more H atoms can be replaced by --F and/or --Cl, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR3## with the provisothat only one of the radicals R¹, R² can be hydrogen, --F, --Cl, --CF₃,--OCF₃ or --CN;

R³, R⁴, R⁵, R⁶ and R⁷ are identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1-16 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, and/or

b2) one or two CH₂ groups can be replaced --CH═CH--,

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system;

M¹, M², M³ and M⁴ are identical or different and are

--CO--O--, --O--CO--, --CO--S--, --S--CO--, --CS--O--, --O--CS--, --CH₂--O--, --O--CH₂ --, --CH₂ --S--, --S--CH₂ --, --CH═CH--, --C.tbd.C--,--CH₂ --CH₂ --CO--O--, --O--CO--CH₂ --CH₂ -- or a single bond;

A¹, A², A³ and A⁴ are identical or different and are

1,4-phenylene in which one or more H atoms can be replaced by F, Cland/or CN, pyrazine-2,5-diyl, in which one or two H atoms can bereplaced by F, Cl and/or CN, pyridazine-3,6-diyl in which one or two Hatoms can be replaced by F, Cl and/or CN, pyridine-2,5-diyl in which oneor more H atoms can be replaced by F, Cl and/or CN, pyrmidine-2,5-diylin which one or two H atoms can be replaced by F, Cl and/or CN,1,4-cyclohexylene in which one or two H atoms can be replaced by CN, Fand/or CH₃, 1,3,4-thiadiazole-2,5-diyl, 1,3-dioxane-2,5-diyl,1,3-dithiane-2,5-diyl, 1,3-thiazole-2,4-diyl, in which one H atom can bereplaced by F, Cl and/or CN, 1,3-thiazole-2,5-diyl in which one H atomcan be replaced by F, Cl and/or CN, thiophene-2,4-diyl, in which one Hatom can be replaced by F, Cl and/or CN, thiophene-2,5-diyl in which oneor two H atoms can be replaced by F, Cl and/or CN, piperazine-1,4-diyl,piperazine-2,5-diyl or 1-(C₁ -C₄)alkyl-1-silacyclohexylene-1,4-diyl;

a, b, c and d are zero or one,

with the proviso that the compound of the formula (I) may contain nomore than four ring systems having five or more members.

The provision of compounds of the formula (I) considerably broadens, ingeneral terms, the palette of liquid-crystalline substances which from avariety of applications-related standpoints are suitable for thepreparation of liquid-crystalline mixtures.

In this connection the compounds of the formula (I) possess a broadscope of application. Depending on the selection of the substituentsthey can be used as base materials forming the predominant part ofliquid-crystalline phase compositions; or alternatively, compounds ofthe formula (I) can also be added to liquid-crystalline base materialsfrom other classes of compounds, in order for example to influence thedielectric and/or optical anisotropy of such a dielectric materialand/or to optimize its threshold voltage and/or its viscosity.

The compounds of the formula (I) are particularly suitable for additioneven in small amounts for influencing the dielectric anisotropy (Δε) ofliquid-crystalline mixtures toward higher negative values.

The novel compounds of the formula (I) are particularly suitable for usein FLC mixtures for ferroelectric switching and/or display devices whichare operated in inverse mode.

Preference is given to compounds of the formula (I) in which

E¹ and E² are identical or different and are --CH--, --CF-- or --N-- andE³, E⁴, E⁵ and E⁶ are identical or different and are --CH-- or --CF--.

Particular preference is given to compounds of the formula (I) in which

E³, E⁴, E⁵ and E⁶ are identical and are --CH--.

Very particular preference is given to compounds of the formula (I) inwhich

E¹, E², E³, E⁴, E⁵ and E⁶ are identical and are --CH-- or in which

E³, E⁴, E⁵, E⁶ are identical and are --CH-- and E¹ and E² are identicaland are --CF--.

Special preference is given to the compounds of the formula (I) in which

E¹, E², E³, E⁴, E⁵ and E⁶ are --CH--.

Preferably, R¹ and R² are identical or different and are

a) hydrogen, --CN or --F,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 18 carbon atoms, where

b1) one or more nonadjacent and nonterminal CH₂ groups can be replacedby --O--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH₃)₂ --, and/or

b2) one CH₂ group can be replaced by cyclopropane-1,2-diyl,1,4-phenylene or trans-1,4-cyclohexylene, and/or

b3) one or more H atoms can be replaced by --F, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR4## with the provisothat only one of the radicals R¹ and R² can be hydrogen --CN or --F.

With particular preference R¹ and R² are identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 16 carbon atoms, where

b1) one or two nonadjacent and nonterminal CH₂ groups can be replaced by--O--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH₃)₂ --, and/or

b2) one CH₂ group can be replaced by 1,4-phenylene ortrans-1,4-cyclohexylene, and/or

b3) one or more H atoms can be replaced by --F, and/or

b4) the terminal CH₃ group can be replaced by one of the followingchiral groups (optically active or racemic): ##STR5## with the provisothat only one of the radicals R¹ and R² can be hydrogen.

Preferably R³, R⁴, R⁵, R⁶ and R⁷ are identical or different and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1-14 carbon atoms, where

b1) one or two nonadjacent and nonterminal CH₂ groups can be replaced by--O--, and/or

b2) one CH₂ group can be replaced by --CH═CH--,

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system.

R³, R⁴, R⁵, R⁶ and R⁷ are, with particular preference, identical ordifferent and are

a) hydrogen,

b) a straight-chain or branched alkyl radical (with or without anasymmetric carbon atom) having 1 to 14 carbon atoms, where

b1) one nonterminal CH₂ group can be replaced by --O--, and/or

c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or --(CH₂)₅ --, ifthey are attached to an oxirane, dioxolane, tetrahydrofuran,tetrahydropyran, butyrolactone or valerolactone system.

Preferably, M¹, M², M³ and M⁴ are identical or different and are

--CO--O--, --O--CO--, --CH₂ --O--, --O--CH₂ --, --CH═CH--, --C.tbd.C--,--CH₂ --CH₂ --CO--O--, --O--CO--CH₂ --CH₂ -- or a single bond.

With particular preference, M¹, M², M³ and M⁴ are identical or differentand are

--CO--O--, --O--CO--, --CH₂ --O--, --O--CH₂ -- or a single bond.

Preferably, A¹, A², A³ and A⁴ are identical or different and are

1,4-phenylene in which one or two H atoms can be replaced by F and/orCN, pyridine-2,5-diyl, in which one or two H atoms can be replaced by Fand/or CN, pyrimidine-2,5-diyl in which one or two H atoms can bereplaced by F and/or CN, trans-1,4-cyclohexylene in which one or two Hatoms can be replaced by CN and/or F, 1,3,4-thiadiazole-2,5-diyl,1,3-dioxane-2,5-diyl, 1,3-thiazole-2,4-diyl, in which one H atom can bereplaced by F, 1,3-thiazole-2,5-diyl in which one H atom can be replacedby F, or thiophene-2,5-diyl in which one or two H atoms can be replacedby F.

With particular preference, A¹, A², A³ and A⁴ are identical or differentand are

1,4-phenylene in which one or two H atoms can be replaced by F,pyridine-2,5-diyl in which one H atom can be replaced by F,pyrimidine-2,5-diyl, trans-1,4-cyclohexylene in which one or two H atomscan be replaced by CN and/or F, 1,3,4-thiadiazole-2,5-diyl,1,3-thiazole-2,5-diyl or thiophene-2,5-diyl.

Very particular preference is given to the following compounds of theformula (Ia) to (Ic): ##STR6## in which R¹, R², M¹, M³, A¹ and A³ havethe meanings and preferences stated above.

The compounds according to the invention are prepared by methods knownper se from the literature, as are described in standard works onorganic synthesis, for example Houben-Weyl, Methoden der OrganischenChemie, Georg-Thieme-Verlag, Stuttgart.

The preparation is carried out under reaction conditions which are knownand suitable for said reactions. Use may also be made here of variantswhich are known per se but which are not mentioned here in any moredetail.

If desired, the starting materials can also be formed in situ, by notisolating them from the reaction mixture but instead immediatelyconverting them further into the compounds of the formula (I).

By way of example, synthesis routes to 6-fluorophenanthridines andcompounds of the formula (I) are indicated in schemes 1 and 2, althoughother methods are also conceivable and possible. ##STR7## Scheme 1

Synthesis of compounds of the formula (I) by boronic acid coupling.R=t-BOC; R'=alkyl, R"=H, alkyl. i) Pd⁰ -catalysis, Tetrahedron Lett.1988, 29, 5463-5466, EP-A 694 530; ii) CF₃ COOH, Tetrahedron Lett. 1988,29, 5463-5466; iii) DAST/CH₂ Cl₂, analogous to EP-A 0 475 444.

An alternative synthesis route (Scheme 2) starts from fluorenones andleads to compounds of the formula (I). ##STR8## Scheme 2

Synthesis of compounds of the formula (I) by Schmidt rearrangement ofcorresponding fluorenones. i) NaN₃ /H₂ SO₄ /CHCl₃, J. Chem. Soc. 1965,3032-3037 or NaN₃ /H₂ SO₄ /CCl₃ COOH, Tetrahedron 1971, 27, 3405-3416;ii) DAST/CH₂ Cl₂, analogous to EP-A 0 475 444.

The reaction of the corresponding oximes with polyphosphoric acid(Beckmann rearrangement) also leads to 6-phenanthridones (J. Chem. Soc.1965, 3032-3037).

The synthesis of the corresponding fluorenones can be carried out, forexample, starting from commercial 2,7-dibromofluorenone or in accordancewith Tetrahedron 1971, 27, 3405-3416.

The positional isomers possibly formed in the rearrangement offluorenones with assymmetric substitution (R^(x) ≠R^(y)) (Scheme 2) canbe resolved subsequently or at one of the subsequent stages by columnchromatography, for example.

The group R^(x) is equivalent to the group R¹ --(A¹ --M¹)_(a) --(A²--M²)_(b) or an appropriate unprotected or protected precursor thereofwhich in later steps can be converted into this group by methods whichare known per se and are familiar to the skilled worker.

The group R^(y) is equivalent to the group (M³ --A³)_(c) --(M⁴ --A⁴)_(d)--R² or an appropriate unprotected or protected precursor thereof whichin later steps can be converted into this group by methods which areknown per se and are familiar to the skilled worker.

The radical R^(x) or R^(y) is synthesized by methods which are known perse and are familiar to the skilled worker. Its preparation takes placeunder reaction conditions which are known and suitable for saidreactions. In this context it is also possible to make use of variantswhich are known per se and are not mentioned in any more detail here.

Reference may be made, for example, to DE-A 23 44 732, 24 50 088, 24 29093, 25 02 94, 26 36 684, 27 01 591 and 27 52 975 for compounds with1,4-cyclohexylene or 1,4-phenylene groups; to DE-A 26 41 724 forcompounds with pyrimidine-2,5-diyl groups; to DE-A 40 26 223, EP-A 0 391203 and German Patent Application 196 48 171.6 for compounds withpyridine-2,5-diyl groups; to DE-A 32 31 462 for compounds withpyridazine-3,6-diyl groups; to EP-A 309 514 for compounds with1,3,4-thiadiazole-2,5-diyl groups, and to EP-A 0 630 903 for compoundswith 1-sila-1,4-cyclohexylene groups.

Reference may additionally be made to Tetrahedron Lett. 1981, 37,2815-1821, ibid. 1985, 26, 61-64 and Z. Chem. 1983, 23, 21-22 for theintroduction of alkyl and acyl substituents, to Bioorg. Med. Chem. Lett.1996, 6, 481484 for the introduction of bromine, chlorine and carboxylsubstituents into positions 2 and 7 of a 9,10-dihydrophenanthrenestructure.

The preparation of disubstituted pyridines, disubstituted pyrazines,disubstituted pyrimidines and disubstituted pyradazines is also given,for example, in the corresponding volumes of the series "The Chemistryof Heterocyclic Compounds" by A. Weissberger and E. C. Taylor (editors).

Dioxane derivatives are judiciously prepared by reaction of acorresponding aldehyde (or a reactive derivative thereof) with acorresponding 1,3-diol (or a reactive derivative thereof), preferably inthe presence of an inert solvent, such as benzene or toluene, and/or inthe presence of a catalyst, for example strong acid such as sulfuricacid, benzene- or 4-toluenesulfonic acid, at temperatures between about20° C. and about 150° C., preferably between 80° C. and 120° C.Primarily suitable as reactive derivatives of the starting materials areacetals.

Some of said aldehydes and 1,3-diols and their reactive derivatives areknown while some can be prepared without difficulty by standard methodsof organic chemistry from compounds known from the literature. Forexample, the aldehydes are obtainable by oxidation of correspondingalcohols or by reduction of nitrites or corresponding carboxylic acidsor derivatives thereof, and the diols by reduction of correspondingdiesters.

2,3-Difluorophenylboronic acid and 4-alkoxy-2,3-difluorophenylboronicacids are described, for example, in WO 96/00710;4-benzyloxy-2,3-difluorophenylboronic acid as well can be obtainedanalogously (WO 96/01246). Examples of 4-alkyl-substituted2,3-difluorophenylboronic acids are given in EP-A 0 363 458.

Compounds in which an aromatic ring is substituted by at least one Fatom can also be obtained from the corresponding diazonium salts byreplacement of the diazonium group with a fluorine atom, for example bythe methods of Balz and Schiemann.

As far as the linking of ring systems to one another is concerned,reference may be made, for example, to:

N. Miyaura, T. Yanagai and A. Suzuki in Synthetic Communications 1981,11, 513-519; DE-C 39 30 663; M. J. Sharp, W. Cheng, V. Snieckus inTetrahedron Lett. 1987, 28, 5093; G. W. Gray in J. Chem. Soc. PerkinTrans. II 1989, 2041 and Mol. Cryst. Liq. Cryst. 1989, 172, 165, ibid.1991, 204, 43 and ibid. 1991, 204, 91; N. A. Bumagin; E. V. Luzikova, I.P. Letskaya, Zh. Org. Khim. 1995, 31, 1657-1662; EP-A 0 449 015; WO-A89/12039; WO-A 89/03821; EP-A 0 354 434 for the direct linking ofaromatics and heteroaromatics; DE-A 32 01 721 for compounds with --CH₂CH₂ -- bridges, and K. Seto et al. in Liq. Cryst. 1990, 8, 861-870 forcompounds containing --C.tbd.C-- bridges.

Esters of the formula (I) can also be obtained by esterification ofcorresponding carboxylic acids (or reactive derivatives thereof usingalcohols or phenols (or reactive derivatives thereof). Another suitableexample of a method for this purpose is the DCC method(DCC=dicyclohexylcarbodiimide; cf.: B. Neises, W. Steglich, Angew. Chem.1978, 90, 556-557).

The corresponding carboxylic acids and alcohols and phenols are eitherknown or can be prepared analogously to known processes.

Particularly suitable reactive derivatives of said carboxylic acids arethe acid halides, especially the chlorides and bromides, and also theanhydrides, including mixed anhydrides, for example, azides or esters,especially alkyl esters having 1-4 carbon atoms in the alkyl group.

Particularly suitable reactive derivatives of said alcohols and phenolsare the corresponding metal alcoholates or phenolates, preferably of analkali metal such as sodium or potassium.

The esterification is advantageously carried out in the presence of aninert solvent. Particularly suitable solvents are ethers, such asdiethyl ether, di-n-butyl ether, tetrahydrofuran, dioxane or anisole,ketones, such as acetone, butanone or cyclohexanone, amides, such asdimethylformamide or hexamethylphosphoramide, hydrocarbons, such asbenzene, toluene or xylene, halogenated hydrocarbons, such astetrachloromethane, dichloromethane or tetrachloroethylene, andsulfoxides, such as dimethyl sulfoxide or sulfolane.

Esters of the formula (I) can also be obtained from the correspondingcarboxylic acid salts (preparation by analogy with DE-C 4304756) byreaction with phenols in accordance with DE-A 4427198.

Ethers of the formula (I) are also obtainable by etherification ofcorresponding hydroxy compounds, preferably corresponding phenols, wherethe hydroxy compound is judiciously first of all converted into acorresponding metal derivative, for example into the correspondingalkali metal alcoholate or alkali metal phenolate by treatment with NaH,NaNH₂, NaOH, KOH, Na₂ CO₃ or K₂ CO₃. This product can then be reactedwith the corresponding alkyl halide, alkylsulfonate or dialkyl sulfate,judiciously in an inert solvent such as acetone, 1,2-dimethoxyethane,dimethylformamide or dimethyl sulfoxide, or else with an excess ofaqueous or aqueous-alcoholic NaOH or KOH at temperatures of betweenabout 20° C. and 100° C.

Also suitable for etherification, for example, is the reaction ofphenols with alcohols using diethyl azodicarboxylate andtriphenylphosphine in accordance with the so-called Mitsunobu method(cf.: O. Mitsunobu, Synthesis 1981, 1-28 or L. Navailles, H. T. Nguyen,P. Barois, Liq. Cryst. 1996, 20, 653-664).

The corresponding alcohols, alkyl halides, alkylsulfonates and dialkylsulfates are either known or can be prepared in analogy to knownmethods.

Regarding the synthesis of specific radicals R¹ and R², reference mayadditionally be made, for example, to:

EP-A 0 355 008 for compounds with silicon-containing side chains,

EP-A 0 292 954 for optically active compounds with an oxirane esterunit,

EP-A 0 263 437 for optically active compounds with an oxirane etherunit,

EP-A 0 361 272 for optically active compounds with a dioxolane esterunit,

EP-A 0 351 746 for optically active compounds with a dioxolane etherunit,

U.S. Pat. No. 5,051,506 for optically active compounds with a2,3-difluoroalkyloxy unit,

U.S. Pat. No. 4,798,680 for optically active compounds with a2-fluoroalkyloxy unit,

U.S. Pat. No. 4,855,429 for optically active compounds with a2-chloroacyl unit,

EP-A 0 552 658 for compounds with cyclohexylpropionic acid radicals,

EP-A 0 318 423 for compounds with cyclopropyl groups in the side chain.

The compounds according to formula (I) are especially useful foroperating in smectic and nematic liquid crystal mixtures, in the case ofnematic mixtures preferably for active matrix displays (AM-LCD) (e.g. C.Prince, Seminar Lecture Notes, Volume I, p. M-3/3-M-22, SIDInternational Symposium 1997, B. B. Bahadur, Liquid Crystal Applicationsand Uses, Vol 1, p. 410, World Scientific Publishing, 1990, E. Luder,Recent Progress of AM LCD's, Proceedings of the 15^(th) InternationalDisplays Research Conference, 1995, p. 9-12) and in-plane-switchingdisplays (IPS-LCD), in the case of smectic liquid crystal mixturespreferably for chirally inclined smectic (ferroelectric orantiferroelectric) displays, for ECB displays (electrically controlledbirefringence) and for electroclinic displays.

The invention also provides for the use of compounds of the formula (I)in liquid-crystal mixtures, preferably smectic and nematic mixtures,especially ferroelectric mixtures. Especially preferred is the use inferroelectric liquid crystal mixtures, which are operated in inversemode.

The invention additionally provides liquid-crystal mixtures, preferablysmectic and nematic mixtures, especially preferred ferroelectric andantiferroelectric, especially ferroelectric mixtures, comprising one ormore compounds of the formula (I).

The smectic and nematic liquid crystal mixtures are preferableapplicable for electrooptic displays, in the case of nematic mixturesespecially for active matrix displays and in-plane-switching displays(IPS-LCD), in the case of smectic liquid crystal mixtures for ECBdisplays, for electroclinic displays and chirally inclined smectic(ferroelectric or antiferroelectric) displays.

The novel liquid-crystal mixtures generally contain from 2 to 35,preferably from 2 to 25 and, with particular preference, from 2 to 20components.

They generally contain from 0.01 to 80% by weight, preferably from 0.1to 60% by weight, particularly preferably from 0.1 to 30% by weight, ofone or more, preferably from 1 to 10, particularly preferably from 1 to5, very particularly preferably from 1 to 3, of the novel compounds ofthe formula (I).

Further components of liquid-crystal mixtures comprising novel compoundsof the formula (I) are preferably selected from known compounds havingsmectic and/or nematic and/or cholesteric phases. These include, forexample:

derivatives of phenylpyrimidine as described, for example, in WO86/06401 and U.S. Pat. No. 4,874,542,

meta-substituted aromatic compounds having a six-membered ring, asdescribed, for example, in EP-A 0 578 054,

silicon compounds as described, for example, in EP-A 0 355 008,

mesogenic compounds having only one side chain, as described, forexample in EP-A 0 541 081,

pyridylpyrimidines as described, for example, in WO 92/12974 and GermanPatent Application 196 48 171.6,

hydroquinone derivatives as described, for example, EP-A 0 603 786,

phenylbenzoates as described, for example, in P. Keller, Ferroelectrics1984, 58, 3 and J. W. Goodby et al., Liquid Crystals and Ordered Fluids,Vol. 4, New York, 1984, and

thiadiazoles as described, for example, in EP-B 0 309 514.

Examples of suitable chiral, nonracemic dopes are:

optically active phenylbenzoates as described, for example, in P.Keller, Ferroelectrics 1984, 58, 3 and J. W. Goodby et al., LiquidCrystals and Ordered Fluids, Vol. 4, New York, 1984,

optically active oxirane ethers as described, for example, in EP-A 0 263437 and WO-A 93/13093,

optically active oxirane esters as described, for example, in EP-A 0 292954,

optically active dioxolane ethers as described, for example, in EP-A 0351 746,

optically active dioxolane esters as described, for example, in EP-A 0361 272,

optically active tetrahydrofuran-2-carboxylic esters as described, forexample, in EP-A 0 355 561, and

optically active 2-fluoroalkyl ethers as described, for example, in EP-A0 237 007 and U.S. Pat. No. 5,051,506.

Suitable additional components of mixtures are set out in particular inthe international patent application WO-A 97/04039, which is expresslyincorporated herein by reference.

Preferred additional components of FLC mixtures operated in the inversemode are:

phenanthrene derivatives of the formula (II), ##STR9## fluoropyridinesof the formula (III), ##STR10## difluorophenylene derivatives of theformula (IV), ##STR11## meta-substituted aromatic compounds of theformula (V), ##STR12## 4-cyanocyclohexyls of the formula (VI), ##STR13##1,3,4-thiadiazoles of the formula (VII) ##STR14## where the symbols andindices have the following meanings: X¹ and X² are identical ordifferent and independently of one another are CH, CF or N,

Y is --F, --CF₃ or --R

R and R' are identical or different and independently of one another areas defined for R¹ and R² in formula (I),

A and M are identical or different and independently of one another aredefined as in formula (I), and

a,b,c,d are identical or different and independently of one another are0 or 1, with the proviso that the compounds may contain no more thanfour ring systems and must, with the exception of the formula (II),contain at least two ring systems.

The mixtures can in turn be employed in electrooptical or fully opticalelements, for example display elements, switching elements, lightmodulators, elements for image processing and/or signal processing or,generally, in the area of nonlinear optics.

In addition the mixtures are suitable for field treatment, i.e. foroperation in the quasi-bookshelf geometry (QBG) (see, for example: H.Rieger et al., SID 91 Digest, Anaheim 1991, 396).

The novel ferroelectric liquid-crystal mixtures are particularlysuitable for operation in the so-called inverse mode or TV.sub.(min)mode (see for example: J. C. Jones, M. J. Towler, J. R. Hughes, Displays1993, 14, No. 2, 86-93; M. Koden, Ferroelectrics 1996, 179,121-129).

Liquid-crystalline mixtures comprising compounds of the formula (I) areparticularly suitable for use in electrooptical switching and displaydevices (displays). These displays are usually constructed in such a waythat a liquid-crystal layer is enclosed on both sides by layers whichare usually, in this sequence starting from the LC layer, at least onealignment layer, electrodes and a limiting sheet (for example of glass).In addition, they comprise spacers, adhesive frames, polarizers and--forcolor displays--thin color-filter layers. Other possible components areantireflection, passivation, compensation and barrier layers andelectric nonlinear elements, such as thin-film transistors (TFTs) andmetal-insulator-metal (MIM) elements. The structure of liquid-crystaldisplays has already been described in detail in relevant monographs(see, for example: E. Kaneko, "Liquid Crystal TV Displays: Principlesand Applications of Liquid Crystal Displays", KTK Scientific Publishers,1987).

The invention therefore additionally provides a switching and/or displaydevice, preferably a smectic or nematic, especially a ferroelectricdevice, comprising a liquid-crystal mixture which comprises one or morecompounds of the formula (I).

In devices, which contain nematic liquid crystal mixtures, active matrixdisplays and in-plane-switching displays (IPS-LCD) are preferred.

In devices, which contain smectic liquid crystal mixtures, ECB displays(electrically controlled birefringence), electroclinic displays andchirally inclined smectic (ferroelectric or antiferroelectric) displaysare preferred.

Such devices are e.g. applicable for computer displays or in chip-cards.

The novel switching and/or display device is preferably operated innormal mode or inverse mode.

Ferroelectric switching and/or display devices operated by multiplexaddressing can inter alia be operated in two different ways, the normalmode or the inverse mode (or TV.sub.(min) mode). The difference betweenthe two is in the addressing scheme and in the various requirementsregarding the dielectric tensor of the FLC material, i.e. the FLCmixture. An overview is given, for example, by J. C. Jones et al. inDisplays 1993, 14, No. 2, 86-93, referred to below as Jones, and M.Koden in Ferroelectrics 1996, 179, 121-129, and the literature set outtherein.

The response characteristics of an FLC device can be generallyrepresented in a diagram in which the driver voltage (V) is plottedhorizontally and the width of the driving pulse (τ, time) is plottedvertically (see for example Jones, FIGS. 4, 8, 10 and 11).

A response curve is determined experimentally and divides the V, τ areainto a switching region and a nonswitching region. Normally, the pulsewidth becomes shorter when the voltage is increased. It is this behaviorwhich characterizes the mode known as normal (see for example Jones,FIG. 4).

With appropriate materials, however, the V τ curve has a minimum (at thevoltage V.sub.(min)), as can be seen in Jones, for example, in FIGS. 8,10 and 11. This minimum comes about as the result of the superimpositionof dielectric and ferroelectric twisting. FLC devices are operated inthe inverse mode if the sum of the row driver voltage and column drivervoltage in the operating temperature range is higher than the minimum onthe V, τ curve, i.e. V.sub.(row) +V.sub.(column) >V.sub.(min).

Express reference is made to the literature references cited in thisapplication, especially regarding the synthesis of compounds of theformula (I); their citation makes them part of the description.

In the present application are cited various documents, e.g. toillustrate the technological surrounding of the invention. All thesedocuments are specially related to this invention, they are part of theapplication by reference.

Also related to the invention is the content of German application No.196 53 009.1, the priority thereof is claimed in this application, andthe abstract of this application; all are part of the application byreference.

The invention is illustrated in more detail by means of the followingexamples, although this is not intended to represent a limitation.

EXAMPLE 1 6-Fluoro-3-octyl-8-hexyloxyphenanthridine ##STR15## From3-bromo-6-fluoro-8-hexyloxyphenanthridine, octylmagnesium bromide and1,3-bis(diphenylphosphino)propane nickel(II) chloride (dppp-NiCl₂) inTHF. EXAMPLE 2 6-Fluoro-8-(2-(S)-fluorooctyloxy)-3-octylphenanthridine##STR16## From 6-fluoro-3-octylphenanthridin-8-ol by reaction with2-(S)-fluorooctan-1-ol by Mitsunobu reaction. EXAMPLE 33-[4-(Butyldimethylsilanyl)-butyloxy]-6-fluoro-8-hexyloxyphenanthridine##STR17## From 6-fluoro-8-hexyloxyphenanthridin-3-ol by reaction with4-(butyidimethylsilanyl)-butan-1-ol by Mitsunobu reaction. EXAMPLE 46-Fluoro-3-octylphenanthridin-8-yl) trans-4-pentylcyclohexanecarboxylate##STR18## From 6-fluoro-3-octylphenanthridin-8-ol by reactingtrans-4-pentylcyclohexanecarbonyl chloride. EXAMPLE 56-Fluoro-8-octylphenanthridin-3-yl) carbonate ethyl ester ##STR19## From6-fluoro-8-octylphenanthridin-3-ol by reaction with ethylchlorocarbonate. EXAMPLE 66-Fluoro-3-(4-hexyloxyphenyl)-8-octylphenanthridine ##STR20## From3-bromo-6-fluoro-8-octylphenanthridine by reaction with4-hexyloxyphenylboronic acid, sodium carbonate andtetrakis(triphenylphosphine)palladium(0) in toluene/ethanol/H₂ O.EXAMPLE 78-(4-Decyloxy-3-fluorophenyl)-6-fluoro-3-hexyloxyphenanthridine##STR21## From 8-bromo-6-fluoro-3-hexyloxyphenanthridine by reactionwith 4-decyloxy-3-fluorophenylboronic acid, sodium carbonate andtetrakis(triphenylphosphine)palladium(0) in toluene/ethanol/H₂ O.

What is claimed is:
 1. A 6-fluorophenanthridine derivative of theformula (I) ##STR22## in which the symbols have the following meanings:R¹ and R² are identical or different and are a straight-chain orbranched alkyl radical (with or without an asymmetric carbon atom)having 1 to 16 carbon atoms, whereb1) one or two nonadjacent andnonterminal CH₂ groups can be replaced by --O--, --CO--O--, --O--CO--,--O--CO--O-- or --Si(CH₃)₂ --, and/or b2) one CH₂ group can be replacedby 1,4-phenylene or trans-1,4-cyclohexylene, and/or b3) one or more Hatoms can be replaced by --F, and/or b4) the terminal CH₃ group can bereplaced by one of the following chiral groups (optically active orracemic): ##STR23## R³ is a) hydrogen, b) a straight-chain or branchedalkyl radical (with or without an asymmetric carbon atom) having 1-14carbon atoms, whereb1) one nonterminal CH₂ group can be replaced by--O--, and/or c) R⁴ and R⁵ together are alternatively --(CH₂)₄ -- or--(CH₂)₅ --, if they are attached to an oxirane, dioxolane,tetrahydrofuran, tetrahydropyran, butyrolactone or valerolactone system.2. A liquid-crystal mixture comprising one or more6-fluorophenanthridine derivatives as claimed in claim
 1. 3. Aliquid-crystal mixture as claimed in claim 2, which is ferroelectric. 4.A liquid-crystal mixture as claimed in claim 2, which contains from 0.01to 80% by weight of one or more 6-fluorophenanthridine derivatives ofthe formula (1).
 5. A ferroelectric switching and/or display devicecomprising a ferroelectric liquid-crystal mixture as claimed in claim 2.6. A ferroelectric switching and/or display device as claimed in claim5, which is operating in the TV.sub.(min) mode.